The proposed research is concerned with the immunochemical investigation of cerebral myelin heterogeneity in the rat and of the cross-reactivity between myelin subfractions and neuroglial cell surfaces. To obtain myelin subfractions for study, a two-dimensional separation scheme will be used -- first to separate unshocked myelinated axonal fragments by density-gradient centrifugation into 3 fractions, then to shock each fraction and separate further into sub-subfractions by the method of Benjamins et al. These 9 subfractions will be obtained from rat brain at 3 different ages -- 20, 40, and 80 days -- to yield 27 in all. The first aim will be to measure the specificities and cross-reactivities of the 27 separate subsfractions in vitro by the method of sequential adsorption analysis using I125-labeled immunoglobulins from antiserums raised against the various fractions, against established astrocytoma cell lines, and against other purified brain components. The second aim will be to measure the immunoglobulins that have interesting properties (on the basis of their in vitro reactions) for their reactions with brain in vivo in rats at different stages of brain development, to look for brain-localized radioantibodies, and to sub-subfractionate the brain containing the localized radioantibodies, according to the original separation scheme. The third aim will be to obtain immunofluorescent and immuno-electron-microscope views of the mode of localization of some of the interesting antibodies in their reactions with cultured cells in vitro, with normal brain in vivo, and with viable myelinating organ cultures of neonatal brain. Immunochemical markers for myelin and for the surfaces of neurological cells will be sought, and, through the patterns revealed by the method of sequential adsorption, monospecific reagent antibodies against the markers will be prepared.